Implantable medical devices are used in a variety of therapeutic applications. In some implantable medical devices, a pulse generator and a medical lead are used together to provide electro stimulation therapy in the form of electrical pulses delivered by electrodes on the medical lead to a tissue site within a patient. Implantable electrodes may be made of alloys of precious metals such as platinum and/or palladium. Such alloys of platinum and palladium (also referred to as platinum and palladium alloys) may be corrosion resistant, biocompatible, and radiopaque. The latter characteristic enables verification during implantation that the electrode is in the proper location within the body for effective therapy delivery. However, platinum and palladium alloys are expensive. Further, platinum and palladium alloys tend to polarize during use to such an extent that the effectiveness of the medical device may be reduced.
Electrode polarization may be reduced by forming an iridium oxide coating on a surface of the electrode. Suitable iridium oxide coatings which may be formed on a platinum or palladium alloy electrodes include anodized iridium oxide films, thermal iridium oxide films, and sputtered iridium oxide films. An anodized iridium oxide film may be formed by depositing a layer of iridium on an electrode surface, and then anodizing the iridium layer in an electrolytic solution to form the iridium oxide film. The layer of iridium may be deposited by sputter deposition, which may be an expensive process that deposits much of the iridium on non-target (e.g., non-electrode) surfaces and requires sputter deposition systems that are expensive to buy and to maintain. A thermal iridium oxide film may be formed by the thermal decomposition of an iridium salt. The process may take days and the adhesion of the resulting iridium oxide film to the underlying surface may be insufficient for medical applications. A sputtered iridium oxide film is formed by direct sputter deposition of iridium oxide onto the surface by sputtering iridium in an oxygen plasma environment. As with the anodized iridium oxide film described above, the sputter deposition of the iridium oxide film may be an expensive process that deposits much of the iridium on non-electrode surfaces and requires sputter deposition systems that are expensive to buy and to maintain. The costly manufacturing operations associated with forming the iridium oxide layer contribute further to the already high cost of the electrodes made of platinum and palladium alloys. Also, subsequent processing of the electrode by, for example, welding, may remove the deposited iridium oxide film and reduce the effectiveness of the electrode.